Fatty acid hydroperoxides and H2O2 in the execution of hypersensitive cell death in tobacco leaves

We initially compared lipid peroxidation profiles in tobacco (Nicotiana tabacum) leaves during different cell death events. An upstream oxylipin assay was used to discriminate reactive oxygen species (ROS)- mediated lipid peroxidation from 9- and 13-lipoxygenase (LOX)-dependent lipid peroxidation. Free radical-mediated membrane peroxidation was measured during H2O2-dependent cell death in leaves of catalase-deficient plants. Taking advantage of these transgenic plants, we demonstrate that, under light conditions, H2O2 plays an essential role in the execution of cell death triggered by an elicitor, cryptogein, which provokes a similar ROS-mediated lipid peroxidation. Under dark conditions, however, cell death induction by cryptogein was independent of H2O2 and accompanied by products of the 9-LOX pathway. In the hypersensitive response induced by the avirulent pathogen Pseudomonas syringae pv syringae, both 9-LOX and oxidative processes operated concurrently, with ROS-mediated lipid peroxidation prevailing in the light. Our results demonstrate, therefore, the tight interplay between H2O2 and lipid hydroperoxides and underscore the importance of light during the hypersensitive response.